Maintaining tire inflation pressure within limits specified by tire and vehicle manufacturers is important for safe and efficient vehicle operation. Underinflated tires may result in poor vehicle handling, reduced fuel economy, reduced tread life, and increased risk of tire failure from frictional heating and puncture. Unfortunately, many vehicles are operated with underinflated tires because vehicle operators may not check tire inflation pressure until they have a visual indication of underinflation. However, by the time a tire is visibly underinflated, tire pressure may already be low enough to cause tire damage, impair vehicle safety, increase tire wear, and decrease vehicle fuel efficiency.
Many devices have been proposed to automatically maintain tire inflation pressure. Some devices couple an air supply through the rotating hub of a vehicle's wheel and inflate or deflate the vehicle's tires in response to measurements of tire inflation pressure. Other devices place a battery-operated pump and pressure transducer inside the air chamber between a tire and a wheel and add air as needed to maintain inflation pressure at a set point value. Although such devices may give very precise control of inflation pressure and may be able to respond to changes in load or road conditions, they are generally incapable of regulating tire pressure without a source of electrical power and are relatively expensive to manufacture, install, and maintain.
Other devices employ a mechanical linkage and a mechanical pressure regulating device to replace air lost from a tire by leakage. The mechanical linkage generates a force related to a change in a separation distance between a reference position, for example a part of the wheel, and a part of an inner peripheral surface of the tire above the flattened contact patch between the tire's tread surface and a road surface. The force generated by the mechanical linkage is used to increase the pressure in a compression chamber. If the resulting pressure in the compression chamber is higher than the air pressure in the tire, an opposing spring force in the pressure regulating mechanism is overcome, a valve opens, and air flows from the compression chamber into the tire.
A separation distance between the inner peripheral surface of a tire and the rotational center of the tire varies with an amount of weight supported by the tire, tire rotation speed, tire inflation pressure, stiffness of tire materials, and other factors. It is therefore difficult to predict the forces generated by and within the mechanical linkage, and automatic inflation devices having mechanical linkages and mechanical pressure regulators have several disadvantages related to the use under commonly encountered operating conditions. Such devices are relatively heavy, making a tire and wheel difficult to rotationally balance. Furthermore, the mechanical linkage and pressure regulator are each subjected to an operating cycle once per wheel revolution, and over the course of a normal service lifetime of a tire, experience millions or even tens of millions of operating cycles. Over such a large number of operating cycles, material properties such as spring constants of components in the pressure regulating mechanism may change from work hardening, self heating, heating from hot road surfaces, heating from vehicle braking system components, corrosion from exposure to water, salt, and other chemicals, or combinations of these effects. As material properties change, the regulated pressure may also change or pressure regulation may fail entirely.
Mechanical wear and dynamic loads from impacts with obstacles or uneven road surfaces may cause some components in a mechanical linkage to bend or break. The air pumping mechanism may fail to operate, or worse yet, metal pieces may be released into the tire, potentially causing sudden, catastrophic tire failure. Or, a mechanical linkage may weaken parts of a tire by repeatedly rubbing or pressing on the tire, possibly leading to tire failure.
What is needed is a device for automatically maintaining tire pressure that is light in weight, simple and economical to install in conventional wheels and tires, reliable, operates without electrical power, does not cause wear on a tire surface, does not includes springs or linkages which may be damaged by friction or impact, and may be adapted to many different sizes of tubeless tires or tires with inner tubes.